This is probably the hardest of the parts of this series to write. And in addition, it goes out in English, because it has a target audience of two (hi guys!) who do not speak Finnish. Not that the hardest part is the language.
Rant about basic and profound philosophical concepts by someone with very little training in philosophy follows. You have been warned.
The one, and only, universal truth is that we cannot know anything. In the end, everything we know is filtered through our senses and our internal representations and interpretations of those sensations. There is no way whatsoever to prove that anything outside my mind exists. In fact, even "I think, therefore I am" is not true — my current feeling of continuity of "my" self could be an illusion, and I would have no way to know. So, the best we can do is "I think, therefore something exists".
While blaming your favourite argument opponent for not realizing the above, in my experience what separates worldviews is not disagreements on it. I do not think I have raised this point with anyone above age 15 who would not have agreed with it, at least in principle. Most fervently religious people I know agree with it as well as do most jealously scientifically-minded atheists. What separates worldviews is what you do with it.
On the very basic level, there is two ways to treat this problem of not knowing. (Well, in addition to totally ignoring it and making a choice without being conscious about it, and then changing your mind as it suits your purposes.)
Either, you can say that since we cannot know, the question of real truth values of beliefs is not meaningful. You can still discuss how useful beliefs seem to be for some specific purpose such as a person’s happiness or his/her experience of being able to predict what happens, but "truth" becomes totally subjective, a question of whether something is true for a particular person. Belief or "inner certainty" constitute proof as good as any — for the person with the certainty, though not necessarily for others.
Or, you can say that while our senses are not reliable, they still "very likely" correspond at least partly to some real, external reality, and then start figuring out what we can (and cannot) guess about that reality given that this basic assumption of it existing and interacting at least with some predictability with our internal realities holds. Of course, if you are honest, the answer boils down to "not a whole lot", but you can still argue it is better than nothing. The concept of "truth" becomes external to the concept of an observer, and at least somewhat testable even if in the end unknowable. Of course, truth-values of a lot of things we experience are still irrelevant (to us, or in general).
This is, naturally, a simplification. But the major point I am trying to make that this is a choice. There is nothing you can experience to say which stance is the true one. The first one has the advantage that it can "explain" the latter — my taking the latter stance is simply my subjective truth, the absolute truth value of which is irrelevant. The second one has the advantage that it actually fits to how we experience the world — it is very hard for a human being to really to work from the assumption that there are no "facts". But it is, in the end, an arbitrary choice.
People have told me that since there is this underlying arbitrary choice, a scientific worldview — which the latter, in my opinion, leads to — is nothing but another religion. I do not think that this is true. It could be true if religions honestly admitted that they take the first choice, but in my experience they rarely do. I suppose most religions developed in conditions where the "what can we really know" was not a relevant question, and so the conscious choice was never made. In addition, I also feel that statement belittles religions. Religions have social, mystical, and historical dimensions that you cannot simply reduce to a "worldview that includes some irrational elements". "A worldview that includes some irrational elements" is simply that — a worldview.
Like you probably have guessed, I have chosen to assume — assume, not believe — that our senses give us very distorted but still somewhat predictable information of a very real external reality, and that the concept of "truth" is not completely subjective. That assumption is arbitrary and I make it partly for simply esthetic preferences. But since I have chosen to make it, I do my best to stick to it.